Skeletal muscle sarcopenia (i.e. reduced muscle mass and strength) is common among sedentary adults and its funtional significance is perhaps most noted in older individuals. This loss of skeletal muscle is largely due to inactivity (i.e. reduced mechanical load), which may impair the growth factor response thought to be important in muscle growth. Recent evidence implicates tissue growth factors, namely fibroblast growth factor (EGF) and insulin-like growth fator-1 (IGF-1), as llikely signals for muscle growth. We have recently reported that bed rest unloading and consequent muscle atrophy are associated with decreased serum EGF (acidic isoform, aFGF) while resistance training (i.e. increased mechanical load) is associated with increased serum aFGF. Further, muscle growth with resistance training is enhanced with increasing amounts of heavy eccentric contractions. The purpose of this study is to compare concentric and eccentric mechanical loading to determine the efficacy of each toward inducing a growth factor (FGF/IGF-1) response. Fifteen healthy subjects at least 19 years of age will perform a single bout of heavy concentric or eccentric exercise with the knee extensor muscle group. Subjects will perform the opposite exercise after a 7-10 day rest period. Before and after each bout, mRNA for IGF-1 will be measured in m. vastus lateralis muscle samples and FGF will be measured in serum. The results will significantly impact the design of exercise protocols for stimulating muscle growth in inactive adults.